What’s Clogging My Boat Slip?

        My husband and I bought our first boat a few years ago, a used 1979, 17-foot Steury stern drive.  Neither of us had prior experience with boat ownership and we looked at several before deciding on this one.  It wasn’t until the boat became ours that we realized the significance of the fact that the seller, just like all the other boat sellers we visited, had a long list of all the new parts that had been installed on the boat in the last year or two.  We thought that was a good thing, not realizing that something goes wrong with boats frequently and they are in constant need of repair.

        So far, we’ve replaced the trim pump, trim pump solenoids, condenser and condenser points, alternator bracket, alternator belt, speedometer pitot and prop.  This year (so far) it was the starter solenoid.  Anyway, with that replaced and the boat good to go, we weren’t exactly good to go.  The boat slip and the channel behind it were clogged with all kinds of weeds.

        I took a look at those weeds and saw free fertilizer for my garden, so I got out my garden rake and lifted slimy piles of them onto the seawall to drain for a day before spreading them around my garden plants.  While I used a garden rake to remove the weeds from the water, there are rakes manufactured specifically for removing weeds from lakes and rivers.  My neighbor graciously let me borrow his, and the rake was strong and lightweight.  Unfortunately the wider-width rake collected so many weeds in one pass that their weight made it too heavy for me to lift it out of the water.

        There was quite a variety of weeds clogging our boat slip.  I tried to identify them all and found that some of them were invasive weeds that shouldn’t be in our waters, while others were native plants that do belong here and provide food for fish, waterfowl, insects, mammals and all kinds of creepy crawlers found in water.  (I looked at a few drops of river water under the microscope a couple years ago and now I don’t want to even stick my toes in the water, much less swim.  It was full of all kinds of squiggly little things.  That microscope ruined my fun!)

        Here are the plants I found in our boat slip.  Except for duckweed, they are submergent plants, meaning that they grow entirely under water, although the leaves sometimes float on the surface.


        There are eight species of milfoil found in Wisconsin and to be honest, many of them are so similar in appearance that I was unable to determine exactly which one was clogging my boat slip.  It doesn’t really matter though – it had to go so our boat could get into the slip. 

        Seven of the milfoil species are native to Wisconsin.  The one bad guy is Eurasian water milfoil which arrived in Wisconsin waters in the 1960s.  It forms dense mats at the water’s surface and thick underwater beds of tangled stems.  Like the other milfoils, it has branching stems with leaves in feathery whorls.

        Besides not being able to get a boat through the mess, you can imagine that no one would want to swim in it or be able to cast a line to fish.  Along with inconveniencing humans, Eurasian water milfoil does even worse to aquatic life.  There is a loss of plant diversity, water quality degrades, and habitat for fish, invertebrates and other wildlife is reduced.

        On the other hand, the native milfoils provide food for waterfowl.  The foliage traps debris that provides food and habitat for invertebrates.  Beds of milfoil provide shade and foraging areas for fish.

Curly-Leaf Pondweed

        The leaves of curly-leaf pondweed remind me of lasagna noodles.  The edges are crimped and appear crispy, but don’t be fooled.  They’re as slimy as any other weed in the river. 

        This invasive plant was accidentally introduced when common carp were stocked in North America.  You probably know how that turned out.  Curly-leaf pondweed grows well in cool temperatures, even under ice.  It peaks around Memorial Day and often dies back entirely by Independence Day.  But it’s not done causing problems for boaters and swimmers when it dies back.  When it dies, it releases nutrients that were held in its leaves and those nutrients cause algae bloom.


        Duckweed is a cute little native plant that floats freely on top of the water, dangling a single threadlike root below.  It reproduces rapidly, often multiplying to large populations in still water.  From a distance, a large spread of duckweed is sometimes mistaken for algae, but a closer look will reveal the petite, bright green, individual oval-shaped plants.  They reproduce by budding so they may be attached in groups of 2-8 plants.

        Duckweed is not attached to the river bottom so it must obtain all of its nutrients from the water through its roots and the undersurface of its leaves.  Since it excels at taking up nutrients from the water, it has been used commercially for wastewater treatment.  The duckweed is then skimmed off periodically.  In fertile water it reproduces very quickly, doubling in numbers in just a few days.

        Duckweed can provide 90% of the nutrition needs for ducks and geese.  Muskrats, beavers and fish also partake.  Large colonies of duckweed provide shade for fish and invertebrates and (best of all?) deter mosquito breeding. 

Wild Celery

        Wild celery’s leaves are ribbon or tape-like and emerge in clusters from the rhizomes (roots).  They have a noticeable stripe down the center and are partially transparent, much like cellophane.  But like all the other river weeds and unlike cellophane, they are slimy.  Just the leaf tips trail along the water’s surface.  It is found in growing in water depths of anywhere from ankle deep to several feet.  It tolerates rough water so it can be found most anywhere.

        Wild celery is a superb source of food for waterfowl.  They eat all parts of the plant from top to bottom.  Canvasbacks have been known to change their fall migration routes to find it.  They are so drawn to wild celery that the Latin name for canvasbacks, Aythya valisneria, comes from the Latin name for wild celery, Vallisneria americana.  The common name comes from the celery-like taste of the meat of ducks which have eaten a lot of the plant.   

        Wild celery is also on the menu for marsh birds and shore birds such as rail, plover, sand piper and snipe.  Muskrats will nibble at it and it provides shade, shelter and food for fish.

Common Waterweed

        Kind of a boring, non-descriptive name, isn’t it?  Though a native plant and not considered invasive, common waterweed can hold its own in the clogging-up-the-boat-slip game. 

        Stems up to three feet long hold lance-shaped leaves that range from a quarter inch to slightly more than a half inch in length.  They whorl around the stem in groups of two or three with the whorls more closely spaced near the stem tips.  Common waterweed produces tiny white three-petaled flowers at the end of thread-like stalks.  The flowers float on the water surface among the tangle of plants. 

        Common waterweed provides shelter and food for fish, although if it gets too dense, fish cannot move through it.  Muskrats and waterfowl eat the plant or the invertebrates that are using the plants as home.

It’s the Law!

The following steps are required by law to prevent the spread of aquatic invasive species:

  1. INSPECT boats, trailers and equipment for plants, animals and debris.
  2. REMOVE all attached aquatic plants and animals.
  3. DRAIN all water from boats, vehicles, and equipment, including livewells and buckets containing fish.
  4. NEVER MOVE plants or live fish away from a waterbody.
  5. DISPOSE of unwanted bait in the trash.
  6. BUY minnows from a Wisconsin bait dealer.
  7. ONLY use leftover minnows when either 1) fishing with them on the same body of water or 2) on other waters if no lake/river water or other fish have been added to the container.

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